From swissblawg to Judgment 1C_59/2015:
In the present decision, the Federal Supreme Court comments on the proof of completeness of information within the meaning of Art. 8 FADP. According to the Federal Supreme Court and a doctrinal opinion, the owner of the data collection is obliged to prove the truth and completeness of his information. According to the Federal Supreme Court, however, in the case of negative facts, the other party is obliged to cooperate more in the provision of evidence, in particular by providing counter-evidence or at least concrete indications of the existence of the fact in question, in this case further data in a data file. The mere assertion that the information is incomplete is not sufficient for this purpose. In the specific case, there were also no serious indications of incomplete disclosure of the file.
See also VPB 67.70.